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``Hedgehog'' magnifier

[Hedgehog photograph]
This photograph shows a ``hedgehog'' magnifier being held under a fluorescent task lamp, with additional ambient light from a window. In practice the lamp's reflection can be reduced by better positioning.

This type of LVA is ``stood'' on the page (``the hedgehog'' has ``feet'' which are not seen when viewed from the top), and it has an aspheric (non-spherical) curvature which reduces image distortion over a larger area than a normal lens-based magnifier does. Some distortion is still present in this picture because I wanted to show the magnifier's effect on a ruler and a pencil, and it was hard to hold it flat over these three-dimensional objects; it's better when rested on a printed page.

The magnifier seems to be made from some kind of acrylic plastic. I once dropped one at an outdoor conference venue; it bounced down a flight of steps (gradient was about 30°) and then an attendant caught it in mid-air like a ball. The impacts had severely limited its visual clarity (I replaced it soon after), but it more-or-less stayed in one piece. Chipping can occur with daily use; I try to keep my best one at home and use another for carrying, as some `bumps' are inevitable.

Magnification factors

The Hedgehog's magnification is advertised as 4.25x (often rounded up to 4.3), which must be an area factor rather than a scale factor (the corresponding scale factor is 2.06; perhaps the sales team preferred to quote the higher number). It's not mathematically possible for unpowered lenses to magnify much more than this unless the viewing area is reduced; video magnifiers can do better but they require power. Fresnel-lens ``full page magnifiers'' don't usually reach the Hedgehog's level of magnification, and lack the ability to double-up as a ``pointing aid'' for readers with gaze-tracking issues (nystagmus etc).

Scale factor 2.06 is enough to make 12-point text look like 24.7-point, and 8-point look like 16.5. (This is why I prefer to use scale factors---you can calculate what it does to font sizes by simple multiplication.)

In this picture, the magnification factor is not constant because (as mentioned above) the magnifier is not being rested flat. The leftmost millimetre seems barely 1.3 times as long as the unmagnified ones, while the scale factor on the far right exceeds 4.3! It's difficult to achieve this in practice however---normally the reader would simply place the magnifier flat on the text and use it as designed to get a fairly constant scale factor of 2.06.

Brightfield?

I'm not sure why these things are called ``brightfield'' magnifiers. Bright field microscopy involves lighting a microscope plate from behind, so that objects appear dark against a light background (field). I suppose brightfield magnifiers are named because they tend to concentrate incident light on the area being magnified; they certainly don't require lighting from below.

The search term "brightfield magnifier" should retrieve some alternatives to the Hedgehog, such as the 1.7x and 2x dome magnifiers made by Schweizer, COIL and MagnaBrite, which can be cheaper if you don't need the extra bit of magnification provided by the Hedgehog, or the Eschenbach stand magnifiers, which slightly exceed the Hedgehog's magnification but are harder to obtain in the UK (and the last one I had tended to scratch at the base because it lacked the Hedgehog's ``feet'', although I'm told Eschenbach have somehow mitigated the scratching issue since then).

The most expensive ones have built-in LED lighting and other perks, but I wouldn't want one of those to be involved in an accident! (Note to marketers: expensive kit is less appealing when you realise it'll sometimes need replacing. There's no such thing as a ``one-off purchase'' if it breaks when dropped. Insurance is not the answer, because we all know those companies do everything they can to avoid paying up. We just have to stick to a price range that we wouldn't mind paying again if there's an incident.)

Obtaining magnifiers

The Hedgehog's real name is Loupes Hautes Performances (LHP) T4, but in the UK it's easier to find by its nickname "hedgehog magnifier". (The embossed logo says T4; the Brevete SGDG mark means ``patented without government guarantee'' and is not part of the product name.)

People who are registered blind or partially sighted may be exempt from VAT and postage costs on specialist magnifiers. In the past I've bought Hedgehogs from Optima in Totnes. They're also listed by QuickTest and Edward Marcus Ltd (IC Online), and a general search for "hedgehog magnifier" (or "brightfield magnifier" for other types) might show if new suppliers have appeared, but I don't know what they're like.

I've sometimes been asked where to buy a Hedgehog in East Asia; sorry I don't know, but they're light to take in hand luggage. Perhaps LHP should open a market over there; may I suggest the Chinese name 刺猬放大镜 cìwei fàngdàjìng.

I wrote this page because I sometimes get questions about the Hedgehog I use. Usual disclaimers apply; it does not constitute medical, legal or financial advice.


All material © Silas S. Brown unless otherwise stated.